The present invention relates to a method for fabricating a composite member, and more particularly to a composite structure fabricating method suited to reliably bond a semiconductor pellet to a substrate.
When a semiconductor pellet is bonded to a heat sink or wiring board, a bonding layer is required to have low thermal resistance and high mechanical strength. In a conventional method disclosed in a Japanese patent application No. JP-A-60-214,536 for forming a bonding layer which is low in thermal resistance (hereinafter referred to as "A-method"), a groove is formed at a central portion of the rear surface of a semiconductor pellet, and the rear surface is bonded to a package substrate through a fusible material, to prevent a void from being generated in a bonding layer. Further, according to another conventional method disclosed in a Japanese patent application No. JP-A-61-125,025 (hereinafter referred to as "B-method"), in order to solve a problem that when a thermal strain is produced by changing ambient temperature periodically or by other means, a pellet crack is generated and thus the composite structure has low mechanical strength, a eutectic metal plate having a plurality of through holes is used as a bonding member, and as semiconductor pellet is pressed against the bonding member, to make a bonding layer voidless, thereby preventing the generation of pellet crack.
In the A-method, a groove is formed in the rear surface of a silicon chip, to improve the wettability of the rear surface with a bonding material (for example, an Au-Si alloy). In more detail, gas can readily escape from a bonding region through the groove, and thus the generation of void is prevented.
According to the inventors' experience, however, the A-method can exhibit the above effect only for particular cases, for the following reasons. For example, in a case where a compound semiconductor chip is used in place of the silicon chip, or Pb-Sn solder is used in place of Au-Si alloy, it is difficult for the A-method to produce the above effect. Firstly, the A-method is inapplicable to a device, in which it is not desirable, from the viewpoint of a fabricating process or reliability, to form a groove at a central portion of the rear surface of a pellet. Secondly, in order to improve the wettability for solder, it is necessary to metallize the whole area of side walls of the groove formed in the rear surface of a semiconductor chip, and thus there arise problems that an uneven surface has to be uniformly metallized, and that a portion insufficiently metallized is not wetted well with solder and thus may induce the generation of void.
Further, only for particular cases, the B-method produce the effect of making a bonding layer voidless, thereby preventing the generation of pellet crack and reducing the thermal resistance of the bonding layer. The reason is as follows.
In a case where a eutectic metal plate having a plurality of through holes is used as a bonding member, and a pellet is pressed against the bonding member to make a bonding layer voidless, the surface of the remaining portion of the eutectic metal other than that portion thereof which flows into and fills up the through holes, is usually coated with a native oxide film, and thus the bonding surface of the pellet is not wetted uniformly with the eutectic metal. Further, due to the generation of local stress, the mechanical strength of a bonding region is reduced and the temperature distribution in a bonding layer is not uniform. Thus, the reliability of the bonding processing is degraded.
Further, in a case where fusible materials other than gold containing eutectic metals, for example, Pb-Sn solder whose surface is readily subjected to natural oxidation, is used in the B-method, it is very difficult for the B-method to produce the expected effect.